Many situations exist where it is desirable to determine whether a pipe, container or vessel is filled with liquid to a predetermined point. This is often done by gauges, float switches or ultrasonic liquid level switches which must penetrate the pipe or vessel to make contact with the liquid. These devices are then used to produce an indication of a liquid level which then may be used to effect some control function such as starting or stopping liquid flow, filling or emptying a tank, etc.
The need for the sensing device to contact the liquid has several disadvantages. For example, there is a need to mount the device in contact with the liquid such as, for example, drilling a hole, making special seals, to bring out gage components, etc.
In U.S. Pat. No. 4,144,517 a liquid level detector is described which utilizes ultrasonic energy by detecting the damping effect on one or more reflections of a longitudinal wave of ultrasonic energy in the wall of a vessel or pipe caused by the presence of a liquid in the interior of the pipe or vessel at the location where the ultrasonic energy was injected. That is, the device of that patent operates to detect the presence of a liquid at a predetermined height in the pipe or vessel by detecting whether or not an interface between the pipe or vessel and the liquid is present. Such a device has several disadvantages in effecting measurements. For example, different wall materials and wall thicknesses have different characteristic impedences and multiple reflections can occur in some instances. Further, the response of such a system can be variable depending upon the wall material. For example, in a pipe or vessel which is of plastic material, practically no reflection occurs and this renders the detection method inoperable. Also, a film of grease or oil on the inner wall of the pipe or vessel dampens the rate of the decay of the multiple reflection. This gives a false indication of a liquid level.